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Pioneer SX-850 Restoration

Discussion in 'Pioneer Audio' started by Ross Henning, Jun 3, 2018.

  1. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I found a decent deal on an SX-850 (decent these days anyway) on eBay. I wish I could find some of the deals that I've read about on this forum, but I'm just not that lucky. The guy who sold it indicated that one of the channels was blown out when he move a speaker while it was playing and the wires crossed. Well when I got it, I found both channels had most of the output transistors blown, so the one channel out advertisement was perhaps a bit misleading. I've got the output transistors on order from Mouser. I've pulled the heatsink and the power amp board out of the chassis so that I could work on them more easily. On the power amp board I've replaced all of the electrolytics and the "greenies". I also replaced the 2SA726 transistors as I've read that these tend to get noisy over time. I also replaced the 150ohm 2watt resistors as a couple of them looked scorched. I've check all of the other components and everything looks fine, however, there is a little scorching on the underside of the PCB where two of the driver transistors are located. The transistors tested fine. I have all of the replacement transistors (based on Mark the fixer's list) on-hand to change out anything on this board, and I'd like to do whatever is necessary before I install it back in the chassis. My question is, should I proactively change out any of these other transistors on the main power amp board, or leave the originals in place? Part of me wants to change them so that they are new and fresh, the other part of me wants to stick with the originals because the replacements are always going to be a little different. Recommendations please.
     

     

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  2. 1MEGΩ

    1MEGΩ Active Member

    Messages:
    222
    Location:
    Mamou, Louisiana
    Another thing would be to make sure the STV4's are good.
     
  3. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I did check those...they should test just like a regular diode, correct? I actually have them each wrapped in bubble wrap while I have the amp board out of the chassis. I know that those are absolutely impossible to replace.
     
  4. Awesomeaudio

    Awesomeaudio AK Subscriber Subscriber

    Messages:
    1,160
    Location:
    Sydney, Australia
    Good thinking on the bubble wrap idea.
    Replacements are rare, if ever found, and are very easily broken.
    Keep the leads straight to avoid accidentally bending them. I'd tape a match stick over the stv and leads during the refurb to avoid that one clumsy moment that will have you saying DOH !!!
    All the best with the 850 :thumbsup:
     
  5. tsd71

    tsd71 RIP Tom Petty

    Messages:
    2,574
    Location:
    New Jersey
    while you are in there change all of the transistors and dont forget the trimmer resistors
     
  6. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    Yep, I got those already. In fact, one of them was burned and partially melted. Does anyone have an opinion on whether or not to replace the drivers and other transistors , or leave the originals?
     

     

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  7. rcs16

    rcs16 Super Member

    Messages:
    4,077
    Location:
    Baldwin, Ontario, Canada
    I'd say leave the drivers as originals. Might want to touch up the solder joints, re-paste, if possible.
    It was my o/p bjts that blew many moons ago in the 950. We liked it loud those days. IMO changing them depends on if you plan to blast the amp or just use it for casual listening.
     
  8. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I’ll either be giving this one to my son, or selling it once it’s restored. I want it to be reliable. For that reason, I tend to want to replace, but on the other hand. I don’t want to ruin the sound quality.
     
  9. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I'm stuck on this SX-850, and I'm hoping Watthour or someone else might be able to give me some ideas of what to try next. I ended up replacing all of the transistors on the amp board just to make sure this thing is super-reliable. I'm running it off of my variac right now to check things, and both channels have high DC voltage (relative...right now "high" is 5.9volts with my Variac set very low) where they shouldn't. For example, the base of Q3 and Q4, as well as the collectors of Q5 - Q8. I've replaced all of the electrolytics, and I've tested every single resistor from one of the channels out of circuit. I've also tested the diodes for leakage. Everything on the board checks out. I've even re-checked the transistors. I thought maybe I was getting better at troubleshooting as I brought an SX-780 back from the dead without any assistance, and based on lessons learned from Watthour on my SX-880. However, I'm stumped on this one as there's nothing else on this amp board that I can find wrong. Could something on one of the other boards that are connected to the amp board be causing this?
     
    Last edited: Jun 7, 2018
  10. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    Just a minor update, I disconnected the leads that run to the protection circuit to see if any voltage was feeding back from that board. No change. I checked the lines coming from that board and no appreciable voltage is coming from that direction. I'm still stumped as ever.
     
  11. Watthour

    Watthour Electron Rancher - JS3600

    What is the present bias current (millivolts across 12/13, 27/28 = milliamps in this case) in each channel?
     

     

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  12. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I will check it and post back, but as I'm currently running it at a very low setting on my Variac, I'm not sure if that will tell you what you need to know. Am I wrong? Should I go to full voltage to check it?
     
  13. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I ran to check right after posting my question on the Variac. Even at the low voltage (I have it cranked up around double now from were I was before), it's showing right around 30mv on both sides.
     
  14. Watthour

    Watthour Electron Rancher - JS3600

    Can it be adjusted at all?
     
  15. tsd71

    tsd71 RIP Tom Petty

    Messages:
    2,574
    Location:
    New Jersey
    do not go full voltage yet, can you adjust the bias and offset with the trimmers?
     
  16. Watthour

    Watthour Electron Rancher - JS3600

    (I think we're barking up the same tree.)
     

     

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  17. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I just checked it, and neither the voltage offset, nor the bias trimmer does anything to the 30mv.
     
  18. Watthour

    Watthour Electron Rancher - JS3600

    My guess is something is awry with the drivers and/or outputs. Double-check the pinouts and lead orientation. Check the connections/sockets for the outputs.
     
  19. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I just went in and double checked all of the drivers and output transistors to make sure I have their pinouts correct. I can't find anything wrong with that. I don't know if this helps, but it appears that the problem still exists with Q10 and Q12 pulled. Even with those two pulled, I get positive voltage (almost matching pin 22) at the collectors of Q6 and Q8 where these should be a very small positive voltage at the collector of Q6 and a matching negative voltage at Q8. Does this provide any clues as to what to check next?
     
  20. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    A little more information, just in case this might help...
    With the variac still turned down, and Q10 and Q12 still removed from the circuit board, here's some voltage measurements:

    Pin 2: -16.18
    Pin 22: 15.37
    Pin 19: 10.47

    Q2
    B: -105mv
    C: -15.7
    E: 0.437

    Q4
    B: .099mv
    C: -16.18
    E: .43

    Q6
    B: -15.78
    C: 14.16
    E: -16.18

    Q8
    B: 14.7
    C: 15.32
    E: 15.34
     

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